Project Summary/Abstract Most human tumors are either inherently insensitive to chemotherapy or acquire resistance upon relapse. Apoptosis is a major cytotoxic mechanism of chemotherapy, and defective apoptosis regulation in cancer cells contributes to therapeutic resistance. Apoptosis-based anticancer agents, such as BH3 and SMAC mimetics, have shown promising results in recent preclinical studies. However, the mechanisms of chemotherapy- induced apoptosis remain poorly understood. The agents in current clinical development have weak efficacy and limited applications due to their target limitations. Our long-term objective is to understand how apoptosis is initiated in human cancer cells, and develop more effective anticancer agents accordingly in a rational way. The ongoing studies have elucidated an essential role of PUMA, a BH3-only proapoptotic Bcl-2 family member, in chemotherapy-induced and p53-dependent apoptosis. Induction of PUMA by anticancer agents is often defective in cancer cells due to p53 deficiencies. Deletion of PUMA led to chemoresistance, and elevated PUMA expression elicited profound apoptotic and antitumor effects. These activities of PUMA are dependent upon its unique BH3 domain, and ability to induce mitochondrial dysfunction through the mitochondrial apoptogenic protein SMAC. Furthermore, PUMA is also activated by targeted anticancer drugs through p53- independent mechanisms. These observations suggest that the potent anticancer activities of PUMA, mediated by a unique BH3 domain structure and SMAC-regulated mitochondrial dysfunction, are broadly involved in anticancer therapy and can be manipulated to develop novel anticancer agents. This competitive renewal seeks to: 1) delineate the key determinants of the potent proapoptotic and antitumor activities of PUMA; 2) determine the roles of PUMA and SMAC in modulating the response to targeted therapy; and 3) identify novel small-molecule PUMA inducers to kill p53-deficient cancer cells. These studies will provide new insight into the anticancer effects of chemotherapeutic agents. In the long run, the results of these studies may lead to improved therapeutic strategies and more effective anticancer agents.